I wonder how you can coordinate the heat from multiple cards for that purpose? I guess you could use a liquid cooling setup for CPUs, but pipe the coolant through a copper coil of pipe inside the tank. Our bathtub-sized tank uses 3,600 watts but you would probably only need 1/3 of that. The hardest part I suppose would be that most tanks are mains pressure tanks these days so it'd be an engineering challenge. Not impossible 'tho.